1. Field
The present invention relates to a mobile robot, and more particularly, to an apparatus and method that corrects an offset value of a gyroscope to correct direction errors of a mobile robot.
2. Description of Related Art
One of the most basic and important functions of a mobile robot, such as a cleaning robot, is position detection. A method using a GPS (Global Positioning System) has been used to determine an absolute position of such a mobile robot. Also, a method in which linear and rotation velocities are measured by an encoder and integrated to determine the position of the mobile robot, a method in which the position of the mobile robot is determined using characteristics of walls and ceilings detected by cameras, a method in which an acceleration detected by an acceleration sensor is integrated twice so as to determine X- and Y-coordinates, and a method in which a rotation velocity, that is, output of a gyro sensor is integrated to determine the direction are known for determining a relative position of the mobile robot.
Each of the aforementioned methods has drawbacks. In the case of determining rotation direction, if GPS is used, it is advantageous to determine the absolute position. However, since GPS is not accurate in a confined space such as an inner space of a building, it is not practical to use GPS in a building. Further, even though the encoder is low cost, rotation errors occur due to floor conditions, assemble errors, and slip, for example.
A gyro sensor is a device for measuring angular velocity. Once the measured angular velocity is integrated over time, it is possible to obtain the azimuth of the robot. When the position of the mobile robot is calculated, the mobile robot can have excellent resistance against environment and can be stand-alone due to the gyro sensor. The gyro sensor for measuring the rotation velocity does not generate errors caused by the floor conditions, impacts from the outside, and collision against objects like an encoder does. However, since the direction errors are accumulated due to offsets of the output of the gyro sensor, it is necessary to correct the offset value that is changed as time goes by.
Gyro sensors are subject to what is called “bias drift.” That is, when the mobile robot is stopped, the signal level of the gyroscope is changed due to direction errors that arise over time. A gyro sensor having a low bias drift tends to be expensive. For this reason, since a mobile robot in which an inexpensive gyro sensor is used has a large bias drift, it is necessary to correct an offset value.
Korean Patent Application Publication No. 2004-62038 entitled “GYRO OFFSET COMPENSATION METHOD OF ROBOT CLEANER” discusses offset correction. According to the robot cleaner of this publication, when it is in a gyro offset correct mode, the robot cleaner is stopped for a predetermined time and gyro offsets are collected. Then, an average and standard deviation is calculated by using the collected gyro offsets to calculate a new gyros offset by averaging the values from which gyro offsets corresponding to noises are stochastically excluded. As a result, it is possible to correct direction error of the gyro sensor.
When it is determined that the robot is stopped, gyroscope signals are inputted to the robot and signals corresponding to noises are removed from the gyroscope signals. An average of the rest of the signals is used as a reference bias value, and an angle calculated on the basis of a previous bias is corrected using the new bias value.
When the mobile robot is stopped, it is possible to obtain the most accurate bias offset. Accordingly, the above-mentioned Korean patent application publication is useful. However, the mobile robot needs stop to measure the bias offset. Since rapid movements for quick cleaning is important in a mobile robot such as a cleaning robot, frequent stops may cause the cleaning efficiency to be decreased.